1. Field of the Intention
This invention relates to superconducting material; more particularly, to a superconducting material containing barium, calcium, copper and oxygen; and to a method for making such superconducting Ba--Ca--Cu--O materials.
2. Description of the Related Art
High temperature superconductors (i.e., those which superconduct at or greater than 77K) were first discovered in 1987. Prior to that time the highest superconducting transition temperature (T.sub.c) material discovered since the beginning of research thereto in 1911 was 23K (degrees kelvin) in an intermetallic composition of NbGe which is now referred to as a conventional low temperature Superconductor (LTS). Materials having a high temperature superconducting (HTS) transition to a superconducting state at critical temperatures (T.sub.c) above 77K belong to the cuprate family, and various such cuprates have now been synthesized that exhibit superconductivity. For example, in U.S. Pat. No. 5,578,551, issued to C. W. Chu et al., a method of synthesis of a Hg--Ba--Ca--Cu--O composition as a high temperature superconductor is taught, and this patent is incorporated by reference for all purposes. A critical temperature, T.sub.c, for transition of this material to its superconducting state of up to 135K was disclosed for the HgBa--Ca--Cu--O composition described by the Chu '551 patent. High superconducting transition temperatures have also been disclosed for similar cuprates having thallium (Tl) instead of Hg. However, mercury (Hg) and thallium (Tl) are both toxic and volatile, and high temperature superconductors that do not require, use or involve the hazards of such volatile and toxic elements or compounds are desired.
In a paper entitled "124K Superconductivity in Cu--Ba--Ca--Cu--O" and published in Modern Physics Letters B, Vol. 9, No. 21 (1995) pages 1397-1406 (hereinafter the "Gao et al. paper"), it was stated that a series of materials of a nominal formula A.sub.1 Ba.sub.2 Ca.sub.3 Cu.sub.4 O, where A=(Cu.sub.1-x M.sub.x) with M=Ag or C and x=0, 0.25 or 0.75 was synthesized at pressures of 5 to 6 Gpa and temperatures of 820 to 1200.degree. C. from appropriate mixtures of BaO, CaO and CuO together with AgO to supply Ag or CaCO.sub.3 to supply C. The highest superconducting transition temperature, T.sub.c, observed for this compound type was in an A Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.10+.delta. system wherein A=(CU.sub.0.75 Ag.sub.0.25) and was 124K.
The Gao et al. paper relates that twenty Cu--Ba--Ca oxides within the family A.sub.1 Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.x wherein A=(Cu.sub.1-x M.sub.x) with M being Ag or C and x being 0.0, 0.25 or 0.75 were synthesized under different conditions. Nearly single-phase Cu-1234 [Cu.sub.1 Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.10+.delta. ] was synthesized at about 5 GPa in those samples prepared at temperature of or above 950.degree. C. with a T.sub.c equal to 117K. A superconducting phase with a T.sub.c up to 124K was also observed to exist in four multiphases samples of (Cu.sub.0.75 Ag.sub.0.25)Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.x prepared in a graphite furnace at 5 to 6 GPa, but only at considerably lower temperatures of 820 and 870 .degree. C. In these four samples the 124K transition appeared in both field-cooled (FC) and zero field-cooled (ZFC) susceptibility (Z) and was usually accompanied by steps at about 117K and 80K, which were thought to belong to Cu-1234 and Cu-1223 phases, respectively. It was concluded by Gao et al., based on structural analysis, that slight differences in local structure (crystal-structure or cation-ordering) or stoichiometry (or doping) within the Cu-12(n-1)n family (n=4 or 3), was possibly responsible for the 124K superconductivity.
A superconducting transition with a T.sub.c up to 124K was thus observed in some multiphase samples of the Ag--Cu--Ba--Ca--Cu--O family. The Gao et al. paper also suggest that C-doping may be important to the formation of this 124K phase composition.
There remains a need, however, to identify more precisely the stoichiometric proportions of these cations, and for a method to reliably synthesize them and a need remains to find compounds having a higher transition temperature (Tc) without need for doping with a volatile and/or toxic atomic element.